A Few Ebola Cases Likely In U.S., Air Traffic Analysis Predicts

Air traffic connections from West Africa to the rest of the world: While Guinea, Liberia and Sierra Leone don't have many flights outside the region, Nigeria is well-connected to Europe and the U.S.

PLOS Currents: Outbreaks

Originally published on September 4, 2014 12:51 pm

It's only a matter of time, some researchers are warning, before isolated cases of Ebola start turning up in developed nations, as well as hitherto-unaffected African countries.

The current Ebola outbreak in West Africa has killed more people than all previous outbreaks combined, the World Health Organization said Wednesday. The official count includes about 3,600 cases and 1,800 deaths across four countries.

Meanwhile, the authors of a new analysis say many countries — including the U.S. — should gear up to recognize, isolate and treat imported cases of Ebola.

The probability of seeing at least one imported case of Ebola in the U.S. is as high as 18 percent by late September, researchers reported Tuesday in the journal PLOS Currents: Outbreaks. That's compared with less than 5 percent right now.

These predictions are based on the flow of airline passengers from West Africa and the difficulty of preventing an infected passenger from boarding a flight.

As with any such analysis, there's some uncertainty. The range of a probable U.S. importation of Ebola by Sept. 22 runs from 1 percent to 18 percent. But with time — and a continuing intense outbreak in West Africa — importation is almost inevitable, the researchers told NPR.

"What is happening in West Africa is going to get here. We can't escape that at this point," says physicist Alessandro Vespignani, the senior author on the study, who analyzes the spread of infectious diseases at Northeastern University.

To be clear, the projection is for at least one imported case of Ebola — not for the kind of viral mayhem afflicting Guinea, Liberia and Sierra Leone.

"What we could expect, if there is an importation, would be very small clusters of cases, between one and three," Vespignani says.

But the probability increases as long as the West African epidemics keep growing. And that means U.S. hospitals, doctors and public health officials need to heighten their vigilance.

The same is true for a roster of 16 other nations, from the U.K. to South Africa, which are connected to West Africa through air traffic, Vespignani and his colleagues say.

There's a 25 to 28 percent chance that an Ebola case will turn up in the U.K. by late September. Belgium, France and Germany will have lower risk. "But it's not negligible," Vespignani says. "Sooner or later, they will arrive."

The probability of imported cases in Africa is higher, not surprisingly. There's more than a 50 percent probability Ebola will show up in the West African nation of Ghana by late September, according to the study. Gambia, Ivory Coast, Morocco, South Africa and Kenya are among 11 African countries where Ebola could pop up.

Officials at the U.S. Centers for Disease Control and Prevention had a presentation on the numbers on Tuesday. The CDC has deployed teams of personnel in West Africa to help bring Ebola under control. And here at home, the agency is charged with preparing both the U.S. medical system and the American public for the possibility that the deadly virus could sneak into this country.

Biostatistician Ira Longini from the University of Florida agrees that Ebola doesn't pose a public health threat in the U.S. and other developed nations. But that doesn't mean that preparation isn't urgent.

"We certainly need to make sure that staff and leadership of American medical centers understand the implications of Ebola," says Longini, who also worked on the study. "We need to have diagnostics in place to identify Ebola quickly. We need quite a few local labs to do this and not just rely on sending samples to the CDC. And we need to make sure isolation and quarantine of contacts takes place. If it doesn't, we could have a small cluster of cases."

The analysis by Longini, Vespignani and their colleagues takes into account the number of airline passengers coming from West Africa to various countries. For instance, more than 6,000 a week arrive in Britain from Nigeria, many of them originating in other African countries.

Hundreds to several thousands travel every week from West Africa to France, Germany, Spain, Italy, South Africa, Egypt, Saudi Arabia, India, China and other countries.

The researchers calculated the impact of severe restrictions on flights from Ebola-affected regions. An 80 percent reduction in air travelers would do no more than delay the impact of Ebola by a few weeks. (A 100 percent choke-off of air travel is considered impossible.)

"Unless you can completely shut down the transportation systems, these kinds of efforts will, at best, buy you a little time," Longini says. "And they can be quite counterproductive because you're interrupting the flow of help, goods and services. It can make the epidemic worse in the country that's being quarantined."

The basic problem with confining Ebola is that, like any infectious disease, people can be infected without showing symptoms. In Ebola's case, the average incubation period is 7 days, though it can be longer. That's more than enough time for an infected traveler to land on the other side of the world.

Fortunately, an Ebola-infected person can't infect others unless he's obviously sick. At that stage, the virus can spread by direct contact with the infected person or bodily fluids. On average, each case of Ebola infects about two other people. That spread rate is similar to that of the flu, and roughly half the rate of smallpox.

Vespignani, from Northeastern University, says screening airline passengers is not going to prevent Ebola from traveling across the globe. "I don't trust screening too much," he says. "It's difficult. Intercepting passengers that are really not sick is not easy."